This invention relates to automatic detecting of through-hole voids in a printed wiring board of multi-layer structure and more particularly to a through-hole void detect method and apparatus suitable for detecting through-hole voids in a multi-layer printed wiring board with high reliability.
In order to electrically interconnect layers in a multi-layer printed wiring board, a film of, for example, copper is formed on the inner wall of a through hole through an electroless plating process. A solder film is then formed on the copper film through an electrolytic plating process.
In a multi-layer board, the diameter of the through hole becomes small and the depth becomes large in proportion to an increase in the packing density of parts carried on the multi-layer printed wiring board and an increase in the number of layers. Under these circumstances, the formation of the plating films on the inner wall of the through hole can not easily be achieved. Additionally defective electrical conduction results if the through holes are not plated.
Such a defect is difficult to detect in the initial phase of production unless the defect is detected and corrected, though the defect will result in operation failure which could be fatal.
It is important to detect and remove defects of abnormal deposition of plated copper on the inner wall of a through hole in the initial stages of production. Defects of this type are hereinafter referred to as through-hole void defect.
In the past, a through-hole void check detecting method as disclosed in, for example, JP-A-60-85596 had been proposed in which a photosensitive plate or film is bonded to one surface of a board to cover substantially all through holes to be checked. The one surface of the board is then exposed to light in such a way that light illumination can pass through all of the through holes to be checked. Finally, the thus treated photo-sensitive plate or film is used as a light-shielding mask to perform through hole check.
According to another reported method disclosed in, for example, Processing Technical Program Natl, Electron Packaging Production Conference, Vol. 1974, pp 56-62 (1974), the layer material is mixed with a luminescent material. A luminescent beam, generated at a through-hole void under the irradiation of ultraviolet ray, is observed or measured. More specifically, the layer material is mixed with a luminescent coloring matter which can be excited by an exciting beam of 350 mm wavelength to generate or luminesce with a luminescent beam of 472 mm wavelength. A filter for passing a beam of 350 mm wavelength is supported above an ultraviolet ray lamp. A printed wiring board to be checked is supported horizontally above the filter and a second filter for absorbing the 350 mm wavelength ultraviolet ray is supported above the printed wiring board. The lower filter, printed wiring board and upper filter are suitably spaced apart from each other.
When the ultraviolet ray emitted from the lowermost ultraviolet ray lamp irradiates the lower filter, in the presence of a through-hole void in a through hole, the ultraviolet ray impinges upon a portion of the printed wiring board by way of the through-hole void to generate a luminescent beam. The operator viewing the upper filter from above can see the defective through hole with the through-hole void shine bright and can detect the through-hole void.
With this prior art method, only relatively large through-hole voids can presumably be detected.
The former prior art method faces the following problems.
First, one end of a through hole must be coverd completely in order that the illumination light can irradiate only the periphery of the through hole and can be prevented from entering the through hole. If one end of the through hole is covered incompletely, the presence of a through-hole void is erroneously recognized in spite of the fact that no through-hole void actually exists. In addition, when the number of through holes is large, troublesome open/close operation results.
Second, the light irradiating the periphery of the through hole must reach to the through hole by way of the interior of the board and the through hole void.
However, when the number of layers is large, the illumination light may be interrupted. In addition, the illumination light may also be intercepted by a circuit pattern which has already been formed on the outermost surface of the printed wiring board. As a result detecting of through hole voids often becomes impossible.
Last, when illumination light irradiates the other surface of printed wiring board opposite to the surface on which the photosensitive plate or film is mounted, it is difficult to prevent the illumination light from entering a through hole.
As regards the latter prior art, it was announced more than ten years ago but has not been adopted as a general method yet.
First, the layer material has to be mixed with the luminescent material. The additional step needed for mixing increases the cost and adversely affects the reliability of the layer material.
Second, the check is directed to a printed wiring board in which the through hole has a ratio of about 1:1 between its diameter and depth. It it is difficult for this prior art method to detect through-hole voids in a present-day printed wiring board in which the board has a thickness of several of millimeters, far larger than the diameter of the through hole being 0.3 to 0.5 mm.
In such a printed wiring board having small-diameter through holes, intensity of a luminescent beam passing through a small through-hole void is very small. Thus, the latter prior art method has difficulties in detecting through-hole voids.